Bar codes are areas of readable printed matter consisting of vertical bars and spaces whose varying widths encode sets of characters, including numbers. These bar codes are used for product identification extensively in many industries, particularly the grocery and food processing industry. With these bar codes designating the products, the products can be easily identified electronically both in warehouses and in commericial outlets. Furthermore, a central computer can automatically price these products at a check-out counter once they are identified. The ease and speed these bar codes introduce to a commercial transaction greatly encourage their wide-spread use. Coupled with the wide-spread use of bar codes, however, is the need for bar code readers that are accurate, that is, equally sensitive with varying ambient light, and low-powered. A typical bar code reader in the prior art, unfortunately, do not have both low-power consumption and accuracy.
A bar code wand reader typically is a device which uses a light source directed from within to create an imaged spot on the target bar code tag. Along with the light spot, an internal photodetector is imaged. This imaged photodetector senses the presence or absence of reflected light from the bar code as the spot moves across spaces and bars. The output of the bar code reader then is processed and decoded, usually by some processor, and the information derived from the processor is then put to use. The accuracy of the information, as can be readily seen, is directly dependent on the sensitivity of the bar code reader. For example, in high ambient light, the bar code reader must be sensitive enough to discern the imaged spot light from the ambient light in order to maintain efficient reolution for the decoding process. Unfortunately in the prior art, a bar code reader generally changes resolution with the amount of ambient light; the greater the ambient light is, the less resolution the bar code reader has. In fact, the typical prior art bar code wand reader overloads in the sun and is then not able to read a bar code. The present invention overcomes this limitation of the prior art. In addition, the present invention requires very low power, typically a tenth of the power consumed by prior art bar code readers.
In the preferred embodiment of the invention, a light source of pulse-modulated light of no more than 3% duty cycle is used to create the image spot. The image spot, comprising of reflections of pulse-modulated light, is received by a photodetector diode, which, in turn, produces an image signal in response thereto. This received image signal, or detected bar code signal, is then amplified by an initial stage and is passed through a high pass filter to remove base band bar code frequencies caused by ambient light. It is again amplified and filtered in a second stage comprising a wideband, low power amplifier. The output from the second stage is peak-detected by a buffer transistor detector rather than a diode as in the prior art; it would otherwise unduly load the circuit. The detected image signal is then finally digitized. Before digitizing the peak-detector output, the signal can be further filtered through a low pass filter for a smoother signal. The digitized output signal from the digitizer stage represents the detected bar code signal and is the final output from the the bar code reader circuit. This output signal can then be further processed for information retrieval.